Added [buildenv] Field Generation

[jelin-sh]

Modified the detect_host_profile function in conan_provider.cmake to support the generation of the [buildenv] field. This enhancement allows for a closer integration with the compilers configured in CMake, thereby improving usability for cross-compilation and cross-platform scenarios.

[CLAassistant]

All committers have signed the CLA.

[memsharded]

Thanks for your contribution @skjsnb

We need to check what would be the best, most general approach to this, but I think it is worth to pursue this improvement.

[memsharded]

Now Conan provides a compiler_executables configuration that might be a better mechanism to define this (it can work even with build systems that will not listen to CC/CXX env-vars). Setting the env-vars might not be necessarily bad, but we need to discuss the best approach.

[jcar87]

Please note that as of #566 - the compiler_executables configuration variable is set and propagated - this should negate the need for CC and CXX. Note that this needs CMake to be aware of both compilers (which is not always the case), so requires both to be specified (either by defining CMAKE_<LANG>_COMPILER, or letting CMake find it if it is defined as a language in the project() call or an enable_language().

The linker is more tricky - since in a lot of scenarios CMake performs linking via the compiler, and CMAKE_LINKER is, to the best of my knowledge, not documented. For cases where it is necessary to define these (via [buildenv] or similar, I would suggest looking at the just-merged feature of customizing the profile:
https://github.com/conan-io/cmake-conan/tree/develop2#customizing-conan-profiles